NAME
semctl
—
semaphore control operations
SYNOPSIS
#include
<sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
int
semctl
(int
semid, int semnum,
int cmd,
union semun arg);
DESCRIPTION
The
semctl
()
system call provides a number of control operations on the semaphore
specified by semnum and semid.
The operation to be performed is specified in cmd (see
below). arg is a union of the following fields:
int val; /* value for SETVAL */ struct semid_ds *buf; /* buffer for IPC_{STAT,SET} */ u_short *array; /* array for GETALL & SETALL */
The
IPC_SET
and
IPC_STAT
commands is defined as follows in
<sys/sem.h>
:
struct semid_ds { struct ipc_perm sem_perm; /* operation permissions */ struct sem *sem_base; /* semaphore set */ u_short sem_nsems; /* number of sems in set */ time_t sem_otime; /* last operation time */ time_t sem_ctime; /* last change time */ };
The
<sys/ipc.h>
and looks like
this:
struct ipc_perm { uid_t cuid; /* creator user id */ gid_t cgid; /* creator group id */ uid_t uid; /* user id */ gid_t gid; /* group id */ mode_t mode; /* r/w permission (see chmod(2)) */ u_short seq; /* sequence # (to generate unique msg/sem/shm id) */ key_t key; /* user specified msg/sem/shm key */ };
semctl
()
provides the following operations:
GETVAL
- Return the value of the semaphore.
SETVAL
- Set the value of the semaphore to arg.val.
GETPID
- Return the pid of the last process that did an operation on this semaphore.
GETNCNT
- Return the number of processes waiting to acquire the semaphore.
GETZCNT
- Return the number of processes waiting for the value of the semaphore to reach 0.
GETALL
- Return the values for all the semaphores associated with semid.
SETALL
- Set the values for all the semaphores that are associated with the semaphore identifier semid to the corresponding values in arg.array.
IPC_STAT
- Gather statistics about a semaphore and place the information in the
semid_dsstructure pointed to by arg.buf (see above).
IPC_SET
- Set the value of the sem_perm.uid, sem_perm.gid and sem_perm.mode fields in the structure associated with the semaphore. The values are taken from the corresponding fields in the structure pointed to by arg.buf. This operation can only be executed by the superuser, or a process that has an effective user ID equal to either sem_perm.cuid or sem_perm.uid in the data structure associated with the message queue.
IPC_RMID
- Remove the semaphores associated with semid from the system and destroy the data structures associated with it. Only the superuser or a process with an effective UID equal to the sem_perm.cuid or sem_perm.uid values in the data structure associated with the semaphore can do this.
The permission to read or change a message queue (see semop(2)) is determined by the sem_perm.mode field in the same way as is done with files (see chmod(2)), but the effective UID can match either the sem_perm.cuid field or the sem_perm.uid field, and the effective GID can match either sem_perm.cgid or sem_perm.gid.
RETURN VALUES
For the GETVAL
,
GETPID
, GETNCNT
, and
GETZCNT
operations, semctl
()
returns one of the values described above if successful. All other
operations will make semctl
() return 0 if no errors
occur. Otherwise -1 is returned and errno set to
reflect the error.
ERRORS
semctl
() will fail if:
- [
EPERM
] - cmd is equal to
IPC_SET
orIPC_RMID
and the caller is not the superuser, nor does the effective UID match either the sem_perm.uid or sem_perm.cuid fields of the data structure associated with the message queue. - [
EACCES
] - The caller has no operation permission for this semaphore.
- [
EINVAL
] - semid is not a valid message semaphore identifier.
cmd is not a valid command.
- [
EFAULT
] - arg.buf or arg.array specify an invalid address.
- [
ERANGE
] - cmd is equal to
SETVAL
orSETALL
and arg.val or the values in arg.array are greater than the system-imposed limit.